Biomethanation, or methanogenesis, is a scientific process whereby anaerobic microorganisms in an anoxic environment decompose biodegradable matter (Dickerson et al. 2009). It occurs naturally in swamps, peat bogs, lakes, ponds, hot springs, and the intestines of ruminants (IEA 2005). Anaerobic digestion (AD) of organic material, another way of terming the above process, can be applied in mechanisms called biogas digesters to produce commercially applicable, stable compounds. The two outputs from anaerobic digestion are digestate, a nutrient-rich, sterile, organic sludge that can often be applied as a fertilizer, and more importantly, biogas (NNFCC 2009). Biogas, a mixture consisting of approximately 60 percent methane (CH₄) and 40 percent carbon dioxide (CO₂) (Hessami et al. 1996), retains upwards of 90 percent of the energy from the initial degraded organic matter (IEA 2005). For this reason, capturing the byproduct of this natural process in biogas digesters represents an efficient potential source of energy. The overall process of anaerobic digestion can be separated into four stages: pretreatment, digestion of waste, recovery of gas byproduct, and treatment of the digestate slurry (Verma 2002). However, before delving into applications and uses of biogas digesters, it is first necessary to gain an understanding of the science at work.

The biological conversion of organic matter in methanogenesis takes place in three different stages and is influenced by a number of different factors (Hessami et al. 1996). In the first stage, hydrolysis, insoluble organic material and compounds like lipids, fats, proteins, and polysaccharides are broken down into soluble monomers, such as amino acids and monosaccharides, which can be used as a source of energy. This stage is enzyme driven and is carried out by strict anaerobes, such as Bactericides and Clostridia, as well as facultative bacteria, such as Streptococci (Yadvika et al. 2004). The second step, which is called acid formation, or acidogenesis, involves the conversion of soluble monomers into volatile fatty acids (Hessami et al. 1996). In this stage, another set of microorganisms ferment the breakdown products into hydrogen, acetic acid, and carbon dioxide (Yadvika et al. 2004). The third stage, methane formation, entails the conversion of these products into biogas and a residual organic sludge (Hessami et al. 1996). The mixture of methane and carbon dioxide that comprises biogas is produced by species of methanogenic bacteria that use acetate, such as Methanosarcina and Methanothrix, as well as other such species that utilize hydrogen and formate, including Methanobacterium and Methanococcus. All of the methanogens that are part of this process are members of the domain Archea, order Methanosarcinalis, which is broken down into two families, Methanosataceae and Methanosarcinacae (Yadvika et al. 2004).

Given this wide range of microbes involved in anaerobic digestion, there are a number of diverse factors that influence the overall nature and speed of the process (Yadvika et al. 2004). Since the many species of microorganisms have different needs and are highly sensitive to a variety of factors, unless carefully controlled, the efficient production of biogas can be sacrificed. More specifically, the species that produce acid differ wildly from those that actually produce the methane, in terms of physiology, nutritional requirements, and responses to environmental conditions. Often, it is the inability to maintain a balance between the needs of these two broad groups of microbes that results in instability of the biogas digester. The inhibition of biomethanation can generally be measured by a decrease in steady biogas production, as well as an accumulation of acids (Chen et al. 2008). 

A number of different parameters affect the performance of biogas digesters. However, monitoring and controlling these factors can enable efficient production. One of the principal challenges to anaerobic digestion is maintaining a desirable pH. According to some literature, the optimal pH is as narrow as the range between 6.8 and 7.2 (Yadvika et al. 2004), while others claim that the range is between 5.5 and 8.5 (Verma 2002). Either way, a relatively narrow pH spectrum is necessary to ensure sustained bacterial growth (Yadvika et al. 2004). A common problem during anaerobic digestion is souring, in which the volatile fatty acids and carbon dioxide yielded by fermentation accumulate, halting methane production, since methanogens have a low tolerance for pH variation (O’Sullivan et al. 2010). However, it is possible to control this pH range through the addition of a simple buffer, like sodium bicarbinate—also known as baking soda (O’Sullivan et al. 2010), or by feeding the organic substrate to the digester at an optimal rate (Yadvika et al. 2004).

Temperature is another factor that plays a significant role in the production of biogas. The diverse array of natural environments in which methane-producing bacteria are found illustrates how biomethanation can take place under a wide range of  temperatures, from 10°C to over 100°C, with moisture content varying from under 50 percent to over 99 percent (IEA 2005). Yet, particular groups of bacteria have optimal temperatures as well as upper limits, above which they will immediately die (Alvarez and Liden 2008). Therefore, the bacterial groups found in biogas digesters may not be able to withstand this broad range. Anaerobic bacteria are most active at mesophilic temperatures between 30 and 40°C and thermophilic temperatures between 50 and 60°C (Yadvika et al. 2004). However, in environments in which sudden and cyclical or regular changes in temperature occur, such as outdoors on a farm, especially in colder climates, it has been shown that biogas can still be produced in a fairly consistent manner, since the methanogens can reactivate once temperatures rise. In such instances, the vast majority of the methane that is produced will be emitted during the stage of the cycle in which temperatures are higher (Alvarez and Liden 2008). Particularly in climates with long, cold winters, such as central Ohio, obtaining the desirable temperature range can pose a challenge. However, some previous work has shown how passive solar heating and proper installation of insulation can help mitigate some of the effects of lower temperatures, keeping the range closer to the optimum (Yadvika et al. 2004).

While not as important as temperature or pH, a number of other factors also impact anaerobic digestion. For instance, the ratio of carbon to nitrogen in the feedstock, or organic substrate added to the digester, impacts efficiency. This is because microorganisms have a tendency to use carbon at rates that are 25 to 30 times higher than nitrogen. For this reason, the proportions of C to N should be maintained at a ratio of 20 or 30 to 1 (Yadvika et al. 2004). Ensuring a proper mix can be achieved is possible through a process called co-digestion. While traditionally, anaerobic digestion was carried out using a single substrate, more recently, applications of biomethanation that involve simultaneous digestion of two or more different substrates has been shown to be more efficient. Co-digestion provides a more balanced array of nutrients, enabling better digestion through higher gas yields. For instance, combining whey with poultry manure has been shown to maintain an optimal C to N balance and pH, while digesting solid waste materials from a slaughterhouse with fruit and vegetable waste and manure had similar benefits. Co-digestion can also prevent the buildup of ammonia (Wu 2007), which is produced through the degradation of nitrogenous material, such as urea or proteins. Ammonia has been shown to inhibit biogas production by increasing the mortality rates of methanogens (Chen et al. 2008).

Yet another factor at play in anaerobic digestion is the organic loading rate (OLR), which is the frequency and speed at which the substrate is added to the digester. For each plant of a particular size, there is an optimal rate at which the substrate should be loaded. Beyond this optimal rate, further increases in the speed at which the substrate is added will not translate proportionally to a higher rate of gas production. Agitation, or consistent stirring of the contents in the digester, also plays an important role in determining the amount of biogas produced. Such mixing is necessary on a regular basis to allow maximum contact between the substrate and the microorganisms. This ties into OLR, because if new substrate is added on a daily basis, this alone can be sufficient mixing. At the same time, while stirring is necessary to ensure maximum substrate-microbe contact, increasing the surface area of the substrate also influences gas production. In this way, as the particle size of the substrate decreases, gas production increases. Research thus suggests that physically pre-treating the substrate, for instance by grinding, could allow for lower amounts of organic matter without decreasing the methane yielded (Yadvika et al. 2004).